In general, the central switching facility performs the following functions:
establishing a one-way connection between an arbitrary input time slot in an arbitrary input multiplex link and an arbitrary output time slot in an arbitrary output multiplex link; PA1 establishing a connection between an arbitrary input time slot and M arbitrary output time slots; and PA1 establishing a connection between N time slots in the same frame of an arbitrary input multiplex link and N time slots in the same frame of an arbitrary output multiplex link while leaving unaltered the integrity and the sequencing of the received frame. PA1 switching between a connection unit referred to as "auxiliary equipment" and speech time slots for audio frequency signalling; PA1 broadcasting tones and recorded announcements over a plurality of output time slots simultaneously; and PA1 establishing permanent switching between time slots in a multiplex link transmitting data or semaphore signalling information between one circuit and another or between a circuit and a semaphore station.
A two-way connection between an end A (calling party) and an end B (called party) is constituted in the form of two oneway connections.
Thus, the central switching facility also provides the following functions:
In order to ensure secure operation, the switching facility comprises two identical switching networks.
In addition, transmission quality must comply with CCITT recommendations Q503 to Q513.
The object of the invention is to monitor transmission on a continuous basis for each connection.
Another object of the invention is to check on connections as established.
A further object of the invention is to detect faults.